DFT and ab-initio molecular dynamics of pyrochlore surface dissolution in aqueous media – Effect of cleavage plane and surface atomic vacancies

Abstract

Pyrochlore, as a complex oxide mineral containing the critical niobium element, offers a rich diversity of surface chemical compositions in different cleavage directions. Hence, very little is known about the structure and stability of pyrochlore’s low-index surfaces. Accordingly, this study aims to investigate the hydration behavior of the defect-free and surface-depleted (110) and (111) Ca2Nb2O6F pyrochlore, terminated with F and Ca ions, under implicit and explicit consideration of water molecules using DFT and ab-initio molecular dynamics (AIMD) simulations. Our results revealed that the F&Ca-terminated (110) surface possesses strong hydrophilic characteristics compared to the (111) cleavage plane. The (110) surface becomes hydroxylated on the undercoordinated oxygen and niobium atoms by the dissociative reactions of water at the interface. Our simulations further captured a major intra-surface reconstruction in the (110) direction upon surface ion depletion. On the defect-free (111) surface, water-surface interactions were limited to a single hydrogen bond between each fluoride on the outermost layer and the water’s hydrogen. The ions removal from the (111) surface led to the exposure of Nb atoms and activated water-surface interaction by making the lower position oxygens accessible by the water molecules. The AIMD simulation results suggested that fluorides are prone to diffuse from the surface after solvation, while oxygen loss could also be an active mechanism for surface stability. Later, the insights could serve as the foundation for future studies to tailor advanced pyrochlore flotation procedures.